1. Field of the Invention
The invention relates to a deflection yoke apparatus to be mounted on the neck of cathode-ray tube for projecting a raster on a screen of the tube by scanning with at least one electron beam, comprising
(a) a pair of horizontal deflection coils for generating a magnetic field to deflect the electron beam in a horizontal direction, PA1 (b) a pair of vertical deflection coils for generating a magnetic field to deflect the electron beam in a vertical direction, PA1 (c) a coil separator for electrically insulating between the horizontal and vertical deflection coils and having an expanded part at its front end, and PA1 (d) a funnel-shaped annular deflection core which forms a magnetic path for magnetic flux generated when deflection current is supplied to the horizontal and vertical deflection coils, the magnetic fields deflecting the electron beam inside said core and producing an external magnetic leakage field outside said core. PA1 (e) two auxiliary coils which are arranged at diametrically opposed positions in the vertical direction outside the deflection core to generate a magnetic field with a direction opposed to the direction of said external magnetic leakage field when a horizontal deflection current is supplied to said coils, each auxiliary coil having a magnetic rod of soft magnetic material as a magnetic core for said coil, and PA1 (f) means for engaging the magnetic rods of the two auxiliary coils in magnetic flux coupling relationship with the smaller end of the funnels deflection core.
2. Description of the Related Art
Recently more stringent standards have been introduced for certain types of picture display devices, notably for monitors, with respect to magnetic leakage fields which they may produce around them. An important source of magnetic leakage fields is the pair of horizontal (or line) deflection coils, which may be operated with deflection currents having frequencies of 15.75 kHz to 120 kHz. It is impossible to design a satisfactorily operating deflection yoke apparatus which does not produce a leakage field. If the leakage field is to be eliminated by means of a protective shield, such a shield would only be effective if the combination of the display tube and deflection unit were also shielded on the display screen side. It is true that the magnetic leakage field of a deflection unit is not very strong; at a distance of 50 cm from the front side of a deflection unit for a 110.degree. monochrome display tube the field strength has already decreased to approximately 1% of the strength of the earth's magnetic field, but it is the variation of the leakage field with respect to time which is important. Field variations may cause electromagnetic interferences in other electronic apparatus which causes these electronic apparatuses to malfunction. Also research is being done to establish whether human health is affected by these leakage fields. The time derivative of the field of the deflection yoke apparatus increases with an increase of the line frequencies, and hence with increasingly shorter fly-back periods.
For compensating the leakage field of the pair of horizontal deflection coils a compensation coil can be used which, when energized, generates a compensating magnetic dipole field. This has been proposed in the published EP-A-220 777. This dipole field can be obtained by energizing an auxiliary coil whose turns have been wound around a bobbin in a square form, which coil has the correct number of turns, the correct surface area and the correct orientation. Energization may be effected, for example, by arranging such coil in series or in parallel with the pair of horizontal deflection coils. The compensation field may be obtained alternatively by energizing two auxiliary coils which are positioned on either side of the deflection yoke apparatus, which auxiliary coils have the correct number of turns, the correct surface area and the correct orientation. Also in this case energization may be effected, for example, by arranging the auxiliary coils in series or in parallel with the pair of horizontal coils.
The auxiliary coils are preferably large so as to reduce their energy content.
However, a problem is that many types of display devices (particularly monitors) lack the space to accommodate large auxiliary coils in their correct position. Consequently, relatively small (too small) auxiliary coils must be used, and so the compensation of the leakage fields consumes much (deflection) energy. Moreover, the sensitivity of the pair of horizontal deflection coils is affected if the known auxiliary coils are arranged in series with them. The induction then increases.